A known inkjet print head comprises a number of ejection units, wherein each ejection unit comprises a liquid chamber for holding an amount of liquid. Commonly, the liquid is an ink, such as a solvent-based or water-based ink, a hot-melt ink at an elevated temperature or a UV-curable ink, but the liquid may be any other kind of liquid. Other examples include liquids that need to be accurately dosed.
Each ejection unit of the known inkjet print head further comprises an electromechanical transducer operatively coupled to the liquid chamber for generating a pressure wave in the liquid held in the liquid chamber. A well-known electromechanical transducer is a piezo-actuator, comprising two electrode and a layer of piezo-electric material arranged therebetween. When an electric field is applied by application of a voltage over the electrodes, the piezo-material mechanically deforms and the deformation of the piezo-actuator generates the pressure wave in the liquid. Other kinds of electromechanical transducers are also known for use in an inkjet print head, such as an electrostatic actuator. Hereinafter, the electromechanical transducer may also be referred to as the actuator.
Each ejection unit further comprises a nozzle in fluid communication with the liquid chamber. If a suitable pressure wave is generated in the liquid in the liquid chamber, a droplet of the liquid is expelled through the nozzle. If the liquid is an ink, the droplet may impinge on a recording medium and form an image dot on the recording medium. A pattern of such image dots may form an image on the recording medium as well-known in the art.
A known disadvantage of the above-described inkjet print head is the susceptibility to malfunctioning of the ejection units. In particular, it is known that an air bubble may be entrained in the nozzle or in the liquid chamber. Such an air bubble changes the acoustics of the ejection unit and as a consequence a droplet may not be formed when the pressure wave is generated. Another known cause for malfunctioning is dirt particles (partly) blocking the nozzle. The presence of dirt does not only block the liquid flow, but also changes the acoustics.
It is well-known in the art to sense a residual pressure wave in the liquid. After the generation of a pressure wave, the acoustics of the ejection unit result in a residual pressure wave that damps over time. Sensing and analyzing this residual pressure wave provides detailed information on the acoustics of the ejection unit. A comparison between the acoustics derived from the residual pressure wave and the acoustics of an ejection unit in an operative state enable to derive the operating state of the ejection unit. Moreover, it is known to determine a cause for a malfunctioning state from the residual pressure wave, if a malfunction state is derived.
A disadvantage of the known method for detecting an operating state is the time needed for sensing the residual pressure wave and the time needed for analysis of the residual pressure wave. Due to this relatively long period needed for sensing and analyzing, it is not possible to perform the analysis for each ejection unit after each droplet ejection.
Moreover, even if there would be sufficient time between consecutive droplet ejections, the computational power needed to analyze each ejection unit after each droplet ejection would be so high, that this would not be commercially feasible.